A. Field of the Invention
This invention relates to the field of alerting monitors and particularly coulometric timing systems.
B. Prior Art
Coulometric cells are known having a cathode cup electrode and an elongated erodable anode filament. The filament is disposed in an electrolyte solution and a current is provided which flows through the cell and provides for electrolytic erosion of the anode filament. When the filament initially opens due to this electrolytic erosion, it has a relatively low resistance which resistance increases in time. Such coulometric cells are described for example in U.S. Pat. Nos. 3,355,731; 3,711,751; and 3,769,557. In many timing applications, it has been required that the timing system be of very low cost and of small size as well as highly reliable in the presence of environmental conditions such as humidity and dust or changes in ambient temperature.
In some timing applications, the coulometric cell has been used with a light emitting diode (LED) where the LED is turned on and illuminates at the time the anode filament initially opens as set forth in my copending patent application for Improved Coulometrically Timed Indicator Lamp Circuits Operable From High Voltages, Ser. No. 658,091 filed Feb. 13, 1976 now U.S. Pat. No. 3,999,088. However, the foregoing application has not provided for the LED being a "normally on" indicator and thus be illuminated during the timing or calibration period and become extinguished or turned off at the end of the timing period when the anode filament opens.